Self-healing polymeric materials are systems that after damage can revert to their original state with full or partial recovery of mechanical strength. Using scaling theory, researchers in the Rubinstein Group, as published in Macromolecules, studied a simple model of autonomic self-healing of unentangled polymer networks. In this model one of the two end monomers of each polymer chain is fixed in space mimicking dangling chains attachment to a polymer network, while the sticky monomer at the other end of each chain can form pairwise reversible bond with the sticky end of another chain. The group studied the reaction kinetics of reversible bonds in this simple model and analyzed the different stages in the self-repair process.
The team observed the slowest formation of bridges for self-adhesion after bringing into contact two bare surfaces with equilibrium, very low, density of open stickers in comparison with self-healing. The primary role of anomalous diffusion in material self-repair for short waiting times is established, while at long waiting times the recovery of bonds across fractured interface is due to hopping diffusion of stickers between different bonded partners. Acceleration in bridge formation for self-healing compared to self-adhesion is due to excess nonequilibrium concentration of open stickers. Full recovery of reversible bonds across fractured interface, formation of bridges, occurs after appreciably longer time than the equilibration time of the concentration of reversible bonds in the bulk.
In studying a material that prevents marine life from sticking to the bottom of ships, researchers led by Carolina Chemistry's Joseph DeSimone, have identified a surprising replacement for the only inherently flammable component of today's lithium-ion batteries: the electrolyte.
The work, published in the Proceedings of the National Academy of Sciences, not only paves the way for developing a new generation lithium-ion battery that does not spontaneously combust at high temperatures, but also has the potential to —after recent lithium battery fires in Boeing 787 Dreamliners and Tesla Model S vehicles— renew consumer confidence in a technology that has attracted significant concern.
Scientists in the Johnson Group, in collaboration with researchers from GlaxoSmithKline, as published in Organic Letters, show how a high throughput screening enabled the development of a copper-based catalyst system for the asymmetric hydrogenation of prochiral aryl and heteroaryl ketones that operates at H2 pressures as low as 5 bar.
A ligand combination of (R,S)-N-Me-3,5-xylyl-BoPhoz and tris(3,5-xylyl)phosphine provided benzylic alcohols in good yields and enantioselectivities. The electronic and steric characteristics of the ancillary triarylphosphine were important in determining both reactivity and selectivity.
Michael Corbett, PhD, from the Johnson Group, describes in Angewandte Chemie how dynamic kinetic asymmetric transformations (DyKAT) of racemic β-bromo-α-keto esters by direct aldolization of nitromethane and acetone provide access to fully substituted α-glycolic acid derivatives bearing a β-stereocenter.
The aldol adducts are obtained in excellent yield with high relative and absolute stereocontrol under mild reaction conditions. Mechanistic studies determined that the reactions proceed through a facile catalyst-mediated racemization of the β-bromo-α-keto esters under a DyKAT Type I manifold.
Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. Published in PNAS and chosen as "paper of the month" by The Latest Science, researchers in the Meyer Group describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell.
Solar water splitting into H2 and O2 with visible light has been achieved by a molecular assembly. The dye sensitized photoelectrosynthesis cell configuration combined with core–shell structures with a thin layer of TiO2 on transparent, nanostructured transparent conducting oxides (TCO), with the outer TiO2 shell formed by atomic layer deposition. In this configuration, excitation and injection occur rapidly and efficiently with the injected electrons collected by the nanostructured TCO on the nanosecond timescale where they are collected by the planar conductive electrode and transmitted to the cathode for H2 production. This allows multiple oxidative equivalents to accumulate at a remote catalyst where water oxidation catalysis occurs.
Published in Analytical Chemistry, scientists in the Allbritton Group in collaboration with colleagues from Pharmacology, Biostatistics and Endodontics, and Biomedical Engineering, all at UNC, and the National Health and Environmental Effects Research Laboratory, describe a novel method for the measurement of protein tyrosine phosphatase, PTP, activity in single human airway epithelial cells, hAECs, using capillary electrophoresis.
Their technique involved the microinjection of a fluorescent phosphopeptide that is hydrolyzed specifically by PTPs. Initial results were then extended to a more physiologically relevant model system: primary hAECs cultured from bronchial brushings of living human subjects. The results demonstrate the utility and applicability of this technique for the ex vivo quantification of PTP activity in small, heterogeneous, human cells and tissues.
Presently, there are few estimates of the number of molecules occupying membrane domains. In a collaborative work published in the journal Traffic, researchers in the Thompson Group describe how they, using a total internal reflection fluorescence microscopy (TIRFM) imaging approach, based on comparing the intensities of fluorescently labeled microdomains with those of single fluorophores, measured the occupancy of DC-SIGN, a C-type lectin, in membrane microdomains.
DC-SIGN or its mutants were labeled with primary monoclonal antibodies (mAbs) in either dendritic cells (DCs) or NIH3T3 cells, or expressed as GFP fusions in NIH3T3 cells. The number of DC-SIGN molecules per microdomain ranges from only a few to over 20, while microdomain dimensions range from the diffraction limit to > 1 µm. The largest fraction of microdomains, appearing at the diffraction limit, in either immature DCs or 3 T3 cells contains only 4–8 molecules of DC-SIGN, consistent with the group's preliminary super-resolution Blink microscopy estimates. The article further discusses how these small assemblies are sufficient to bind and efficiently internalize a small (∼50 nm) pathogen, dengue virus, leading to infection of host cells.
Researchers in the Johnson Group, published in JACS, describe an asymmetric total synthesis of the aminocyclopentitol pactamycin. The title compound is delivered in 15 steps from 2,4-pentanedione. Critical to this approach was the exploitation of a complex symmetry-breaking reduction strategy to assemble the C1, C2, and C7 relative stereochemistry within the first four steps of the synthesis.
The article describes multiple iterations of this reduction strategy, and a thorough analysis of stereochemical outcomes is also detailed. In the final case, an asymmetric Mannich reaction was developed to install a protected amine directly at the C2 position. Symmetry-breaking reduction of this material gave way to a remarkable series of stereochemical outcomes leading to the title compound without recourse to nonstrategic downstream manipulations. This synthesis is immediately accommodating to the preparation of structural analogs.
There are many potential benefits of converting biomass to fuels and feedstocks. In particular, the widespread availability and built-in stereochemistry of carbohydrates make them attractive starting materials for the production of value-added products. However, the large number of oxygen atoms relative to the number of carbon atoms renders carbohydrates "overfunctionalized" for many applications.
Researchers in the Gagné Lab recently published a method in Angewandte Chemie for using a commercially available non-metallic catalyst to remove some or all of the oxygen functionalities and replace them with hydrides. Even robust compounds such as methylcellulose are deoxygenated under the optimized conditions. The hydride equivalent is provided by an alkylsilane, and the identity of the silane can be used to control whether complete deoxygenation occurs to give fuel-like hydrocarbons or whether partial deoxygenation occurs, giving feedstock products with well-defined stereochemistry that are difficult to access with currently known methods.